The present invention relates to apparatus and methods of automated wafer-grinding of semiconductor wafers, and more particularly to automatically grinding by monitoring a grinding parameter.
The source material for manufacturing semiconductor chips is usually a relatively large wafer of silicon. Such wafers may be produced by slicing a silicon crystal ingot to a suitable thickness to obtain a number of nearly disk-shaped semiconductor wafers. Both surfaces of each wafer are subjected to abrasive machining, and then etched in a suitable mixed acid solution. One surface of each wafer is then polished to obtain a mirror surface. Circuits are fabricated in the mirror surface of the resulting semiconductor wafer by known processing steps, such as, for example, printing, etching, diffusion, or doping.
When the silicon wafers are sliced from the crystal ingot, the thickness of the wafers is usually greater than desirable for a finished integrated circuit product so as to provide a more robust wafer to stand up to the rigors of the integrated circuit fabrication process. Relatively thick silicon wafers may be necessary, for example, during certain integrated circuit fabrication steps to prevent warpage and breakage of the wafer as a result of heating, handling, and other circuit fabrication processes. Because the thickness of the wafer after the circuit fabrication process is usually greater than desirable for device packaging restrictions, it is typically necessary to grind a backside surface of the wafer opposite from the surface on which the integrated circuits are formed to reduce the wafer thickness.
Automated grinding machines for grinding the backside surfaces of wafers are known. Conventional grinding machines generally include a plurality of chuck tables that secure a plurality of wafers to be ground by one or more grinding wheels. A conventional grinding wheel typically includes a plurality of diamonds embedded in a resinous binder, with some of the diamonds exposed and some unexposed. As the grinding progresses, the exposed diamonds wear down to the level of the binder. The binder is selected to erode during grinding to expose fresh diamonds. The rate of wear of the grinding wheel may be dependent on the composition of the binder, the grinding rate, or other factors, as described more fully below.
FIG. 1 is a side elevational view of an automated grinding machine 10 for grinding a backside surface 25 of a wafer 12 in accordance with the prior art. The grinding machine 10 includes a spindle housing 14 disposed about a spindle 16 having a rotatable grinding shaft 18. A grinding wheel 20 is rigidly secured to the end of the shaft 18. A spindle motor 22 rotates the shaft 18 and the grinding wheel 20 at conventional speeds of 2400-3200 RPM during the grinding process, causing the grinding wheel 20 to grind away semiconductor material from the backside surface 25 of the wafer 12. The spindle housing 14 is coupled to a feed mechanism 26 that allows the placement and the feed rate of the grinding wheel 20 to be adjusted relative to the wafer 14 to provide, for example, different grinding rates.
A controller 27, such as a computer, is electrically connected to the grinding wheel 20 by electrical conductor 29 to receive feedback signals, and to a feed rate motor 31 by electrical conductor 33 to send control signals thereto. The controller 27 is also connected to a shaft speed sensor 19 by electrical conductor 35, to a spindle motor current detector 21 by electrical conductor 37, and to the spindle motor 22 by electrical conductor 23. The wafer 12 is secured to a chuck table platform 30 of a chuck table 28 by a suitable securing mechanism, such as vacuum suction, with the front side of the wafer 12 that includes the integrated circuits positioned against the chuck table platform 30. The chuck table platform 30 is secured to a shaft 32 which is driven by a chuck table motor (not shown) at conventional speeds of between 50-300 RPM.
FIG. 2 is a bottom plan view of the grinding wheel 20 of the grinding machine 10 of FIG. 1. FIG. 3 is a partial cross-sectional radial view of the grinding wheel 20 of FIG. 2. As shown in FIGS. 2 and 3, the grinding wheel 20 includes a disk portion 40 and an annular shoulder 42 depending downwardly from the peripheral edge 41 of the disk portion 40. The annular shoulder 42 includes a lower surface 47. A plurality of cylindrical cavities 44 are formed in the lower surface 47 of the annular shoulder 42 and a cylindrical grinding tooth 46 is disposed in each cavity 44. Each cavity 44 is connected to a central shaft-receiving bore 43 by a pressure signal transmission pathway 45.
As best shown in FIG. 3, each grinding tooth 46 includes a body 48 having a first end 50, which includes a grinding surface 24, and a second end 52. The second end 52 is disposed in the cavity 44. A pressure sensor 54 is disposed in the cavity 44 between the second end 52 and the disk portion 40. The pressure sensors 54 may include, for example, a piezoelectric element 60 that produces an electrical voltage when it is squeezed. Thus, the pressure sensor 54 may convert mechanical pressure on the grinding teeth 46 into an electrical signal, the strength of which increases or decreases with the pressure exerted by the grinding wheel 20 against the backside surface 25 of the wafer 12. The grinding surface 24 may include a plurality of diamonds suspended in a resinous binder. As disclosed, for example, in U.S. Pat. No. 5,827,112 to Ball, incorporated herein by reference, the binder may be selected to be reactive with wheel dressing and to dissolve, either mechanically, or chemically or both. As the binder dissolves, the dull diamonds from the grinding surface 24 are released and washed away, leaving freshly exposed sharp diamonds.
The controller 27 may receive input signals from the pressure sensors 54 to indicate the pressure exerted by the grinding wheel 20 against the wafer 12. The controller 27 may also receive input signals from the speed sensor 19 indicative of the rotational speed of the shaft 18, and input signals from the current detector 21 which indicate the amount of current being drawn by the spindle motor 22. Based on these input signals, the controller 27 may adjustably control various operating parameters of the automated grinding machine 10, including, for example, the feed rate of the feed rate motor 31, the rotational speed of the spindle motor 22, or the release of wheel dressing for sharpening the grinding wheel 20.
FIG. 4 is a schematic view of a typical grind recipe 80 of a grinding machine 10 in accordance with the prior art. During the grinding process shown in FIG. 4, the grinding wheel 20 descends along a z-axis as a function of time t (shown as the horizontal axis in FIG. 4), allowing the grinding teeth 46 to grind away the backside surface 25 of the wafer 12. During a first or xe2x80x9crapid descentxe2x80x9d phase 82, the grinding wheel 20 maintains a relatively high rate of descent between times t0 and t1. During a second or xe2x80x9cF1 removalxe2x80x9d phase 84, the rate of descent of the grinding wheel 20 is decreased (typically 40 microns per minute) between times t1 and t2. Finally, during a third or xe2x80x9cF2 removalxe2x80x9d phase 86, the rate of descent of the grinding wheel 20 is further decreased (typically 20 microns per minute) between times t2 and t3. Thus, in the representative grind recipe 80, the time required to remove a wafer layer of thickness z0-z3 is the time t3-t0. The times t1, t2, and t3 are typically selected to avoid stress cracks or other defects in the wafer 12.
In addition to descent rate of the grinding wheel, other operating conditions of the grinding machine 10 may be varied during the phases 82, 84, 86. For example, the rotational rate of the grinding wheel may be varied, or different grinding wheels having grinding surfaces with different diamond sizes may be used. Grinding machines 10 having grind recipes of the type shown in FIG. 4 typically process approximately 35 wafers per hour.
Various grinding machines have been disclosed to control the forces applied to the wafer. For example, U.S. Pat. No. 5,035,087 to Nishiguchi et al discloses a grinding machine that compares the shaft motor current and a rotation speed of the shaft with predetermined values to derive actual and desired grinding resistance values. The shaft speed is adjusted to bring the actual grinding resistance value closer to the desired value. U.S. Pat. No. 5,545,076 to Yun et al discloses an apparatus for removing dust from a wafer during the grinding process includes a controller for controlling the grinding device and cleaning device. U.S. Pat. No. 5,607,341 to Leach discloses an apparatus for polishing the wafer having a plurality of blocks that move up and down in a grinding wheel. A magnetic fluid is contained in the grinding wheel and cooperates with a magnet disposed below the wafer to apply a force to the blocks. Thus, various methods are known for controlling the grinding force exerted by the grinding wheel 20 on the wafer 12, thereby controlling the grinding rate.
Prior to commencing a grinding procedure, a calibration may be performed with the wafer 12 removed from the chuck table platform 30. The feed mechanism 26 may lower the grinding wheel 20 until the grinding surfaces 24 (FIG. 3) of the grinding wheel 20 contact the chuck table platform 30, providing a xe2x80x9czeroxe2x80x9d or reference position along the z axis (FIG. 1) which may be stored, for example, in a memory of the controller 27. As the grinding wheel 20 is raised, a series of measurements of the distance between the grinding surfaces 24 and the chuck table platform 30 may be made and entered into the controller 27 to create a database of measured calibration data in the memory of the controller 27. Thus, based on a given position of the feed mechanism 26, the controller 27 may determine a xe2x80x9cpredictedxe2x80x9d position of the grinding surfaces 24 of the grinding wheel 20 based on the measured calibration database.
Because the grinding surfaces 24 wear during the grinding process, the predicted position of the grinding surfaces 24 based on the measured calibration data may not accurately reflect the true position of the grinding surfaces 24, particularly after the grinding surfaces 24 have been used for an extended period of time. Generally, the longer the grinding wheel 20 is used, the greater may be the discrepancy between the predicted position of the grinding surfaces 24 determined from the measured calibration data, and the actual position of the grinding surfaces 24. The discrepancy between the predicted and actual positions of the grinding surfaces 24 results in uncertainty over the true thickness of the wafer 12 during the grinding process. For thick wafers, however, the uncertainty over the true thickness of the wafer 12 may be negligible. Alternately, the grinding process may be repeatedly interrupted to manually measure the actual thickness of the wafer 12 until a desired wafer thickness is achieved.
Although desirable results have been achieved using the above-described grinding machines and grinding procedures, the ever-increasing demands of the semiconductor industry for reducing the size of semiconductor chip assemblies are placing unprecedented demands on such machines and procedures to be more accurate. For example, decreasing the size of semiconductor chip assemblies requires decreasing the thickness of the wafer. As wafer thickness is reduced, increased requirements are placed on the grinding machine to more accurately determine the thickness of the wafer and to more accurately control the grinding rate of the grinding wheel 20 against the backside surface 25 of the wafer 12. As wafer thickness is decreased, extra care must be taken to ensure that the wafer is not over-ground or made too thin.
Furthermore, because thinner wafers are more prone to stress cracking or breakage due to the pressure from the grinding wheel, the descent rate of the grinding wheel must be more carefully controlled to avoid damaging thinner wafers. The uncertainty over the actual thickness of the wafer due to the wear of the grinding surfaces may become more important as the wafer thickness is decreased, and may require more frequent interruptions of the wafer grinding process to measure the actual thickness of the wafer. The grinding process is thereby slowed, and the throughput of the manufacturing process is reduced.
The present invention is directed to apparatus and methods of automated wafer-grinding using grinding surface position monitoring. In various aspects of the invention, grinding surface position monitoring may include, for example, monitoring acoustic or optical signals reflected (or through-beam or electrically or magnetically coupled) from the grinding surface, and may be used in combination with monitoring of other operating characteristics, such as grind pressure, shaft speed, or current drawn by a drive motor. Apparatus and methods according to the invention provide improved accuracy and increased throughput of the grinding process.
In one aspect, an apparatus for grinding a working surface includes a grinding surface engageable with at least a portion of the working surface, and a feed mechanism that controllably adjusts a position of the grinding surface. The apparatus further includes a position sensor that senses a position of the grinding surface along an axis approximately normal to the working surface and a controller that receives a position signal from the position sensor and transmits a control signal to the feed mechanism in response to the position signal. In alternate aspects, the position sensor may be an acoustic sensor, an optical sensor, or another type of sensor. The grinding surface may include a grinding material suspended in a binder, the grinding material being worn during grinding.
In an alternate aspect, an apparatus further includes a supplemental sensor that senses an operating characteristic and outputs a characteristic signal. The controller receives the characteristic signal and transmits the control signal to the feed mechanism based on at least one of the position signal or the characteristic signal. In alternate aspects, the characteristic signal may include a pressure of the grinding surface on the working surface, a shaft speed of a drive shaft, a current drawn by a drive motor, or some other parameter.
FIG. 1 is a side elevational view of an automated grinding machine in accordance with the prior art.
FIG. 2 is a bottom plan view of a grinding wheel of the grinding machine of FIG. 1.
FIG. 3 is an enlarged, partial cross-sectional radial view of the grinding wheel of FIG. 2.
FIG. 4 is a schematic view of a typical grind recipe of a grinding machine in accordance with the prior art.
FIG. 5 is a side elevational view of an automated grinding machine having an acoustic sensor in accordance with an embodiment of the invention.
FIG. 6 is an enlarged, partial cross-sectional radial view of the grinding wheel and the acoustic sensor of the grinding machine of FIG. 5.
FIG. 7 is a schematic view of a grind recipe of the grinding machine of FIG. 6 compared with the typical grind recipe of FIG. 4.